Manganese dioxide (MnO2) is a relatively abundant and inexpensive metal oxide. Manganese oxide-based nanoparticles have been used for water purification for a long time (Prasad and Chaudhuri 1995). The redox properties of manganese oxide minerals make them useful catalysts in industrial processes. Naturally occurring manganese oxides have a MnO6 octahedron structure that assembles into a large variety of structural arrangement, yielding minerals with high surface area. Also, multiple oxidation states of Mn atoms in a single mineral facilitate catalysis of oxidation reactions (Post 1999). A great deal of attention has been placed recently on the synthesis of novel MnO2-based catalysts for the removal of formaldehyde and other volatile organic compounds (VOCs) at room or low temperatures (<100° C.) (Sekine 2002; Xu et al. 2008). Xu et al. (2008) used the results of bench scale experiments to predict airborne formaldehyde removal efficiency of coated honeycomb substrates with various dimensions, indicating that up to 20% formaldehyde removal efficiency could be obtained with very low pressure drops (about 2 to 3 Pa) operating at face velocities typical of air filtering systems (1-3 m s−1). In addition to active air cleaning applications, there is some additional evidence of Mn-based catalyst efficacy in passive applications. In a residential setting, deployed manganese oxide within wallboard was reported to reduce 50% to 80% of indoor formaldehyde levels throughout a 7-month long study period (Sekine and Nishimura 2001).
Doping of MnO2 with other transition metals and synthesis of mixed oxides showed improved formaldehyde removal efficiencies. For example, MnOx—CeO2 catalysts had improved performances than MnO2 synthesized by the same method (Tang et al. 2006; Tang et al. 2008). Also, other authors showed good performance of manganese oxides doped with other transition metals, such as vanadium (Tang et al. 2010) and tin (Wen et al. 2009). Several manganese oxide nano and meso structures (e.g., pyrolusite, cryptomelane) have been shown to have very high catalytic activity in the complete oxidation of formaldehyde (yielding CO2 and H2O) at low temperatures, explained by its porosity, degree of crystallinity, reducibility and average oxidation state of the manganese atoms (Chen et al. 2009). Further, nano-structured mixed valence oxides (such as Mn3O4) were shown to effectively catalyze the oxidation of formaldehyde at room temperature (Ahmed et al. 2010). In most of the cases mentioned here, low temperature oxidation of formaldehyde likely takes places via a Mars-van Krevelen (MvK) mechanism, as is usually described for high temperature catalysis, in which lattice oxygen atoms from the catalyst participate in the initial step of the reaction, and are subsequently replenished by reduction of atmospheric O2 (Doornkamp and Ponec 2000; Cellier et al. 2006):MnOx-1—O(s)+CH2O(ads)→MnOx-1(s)+2H(ads)+CO2(g)  (1)MnOx-1(s)+2H(ads)+O2(g)→MnOx-1—O(s)+H2O(g)  (2)
In this mechanism, catalyst efficiency is associated with the number of active surface sites with Mn atoms susceptible to be cyclically reduced and re-oxidized as shown in equations 1 and 2. Formic acid can be formed as a byproduct of incomplete oxidation, and may be found in the gas phase or adsorbed to the catalyst.
Although study exists on the MnO2 catalytic oxidation of formaldehyde there is not enough evidence to support the MvK mechanism. Prior studies also fail to address the issue of implementing this technology successfully to eliminate gaseous formaldehyde at indoor levels.